Bug 1559568 - Make DOMMediaStream support WeakPtr. r=jya

As required by the next patch to avoid strong refs to DOMMediaStream in
OutputStreamManager.

Differential Revision: https://phabricator.services.mozilla.com/D36569

--HG--
extra : moz-landing-system : lando

dom/media/DOMMediaStream.h

@@ -171,7 +171,8 @@ class OverlayImage;
 class DOMMediaStream
     : public DOMEventTargetHelper,
       public dom::PrincipalChangeObserver<dom::MediaStreamTrack>,
-      public RelativeTimeline {
+      public RelativeTimeline,
+      public SupportsWeakPtr<DOMMediaStream> {
   friend class dom::MediaStreamTrack;
   typedef dom::MediaStreamTrack MediaStreamTrack;
   typedef dom::AudioStreamTrack AudioStreamTrack;
@@ -185,6 +186,8 @@ class DOMMediaStream
  public:
   typedef dom::MediaTrackConstraints MediaTrackConstraints;
 
+  MOZ_DECLARE_WEAKREFERENCE_TYPENAME(DOMMediaStream)
+
   class TrackListener {
    public:
     virtual ~TrackListener() {}